Constant-air-volume systems


Use in new systems discouraged by code.

+Q +P — Q +W +Q

Constant-air-volume systems




———— XH1

Winter heating

Constant-air-volume systemsSummer:

Single mixing with room air:

O + R => M (cooling + dehumidification) => C (re-heat) => I (Q + W ) => R Double mixing with room air:

O + R => Ml (cooling + dehumidification) => C’ + R => M2 or I (Q + W ) => R Winter:

O (pre-heat) => HI + R => M (humidification) => H2 (re-heat) => I (Q + W) => R

4.3.1 Variable-air-volume systems

Constant-air-volume systems


подпись: rExample:

You turn the fan speed up or down in your car. Q = m (iR — ii)


AHU fan varies power to match loads. Less load => lower fan power.

Pressure in supply ducts is maintained to a fixed value.

Design cooling:

• box is 100% open

• no reheat

Off-design cooling:

• zone temperature drops since cooling load decreases

• box throttles until minimum flow is reached

Dead band:

• no control action

• start reheat at lower limit

Off-design heating:

• minimum primary air

• thermostat increases reheat as space temperature falls

Design heating:

• fully energized

Constant-air-volume systems

FIGURE 11.23

Vanable-air-volume system with optional reheat.

VAV terminals:

• Single-blade dumper (pressure dependent or independent)

• Air valve

• Induction

Primary flow induces secondary flow from plenum.


Constant-air-volume systems

Constant-air-volume systems

Constant-air-volume systems


подпись: (i)KIGl’RE 17.3 VAV boxes: fa) single-blade. pressure-dependent: (bl single-blade, pressure-»dependent:

:rvJ I Cl :l«r valve.

Constant-air-volume systems

Constant-air-volume systems

FIGURE 11.246

Schematic diagrams of series and parallel fan-powered VAV box.

Constant-air-volume systems

Cutaway drawing of parallel VAV box. (Courtesy of Titus, inc.)

Constant-air-volume systems

{ (<*)



FIGURE 11.25

(a) Flow characteristics for series fan-powered VAV boxes. Cross-sectioned areas represent primary airflow. Total box airflow is constant. (*) Flow characteristics for parallel fan-powered VAV boxes. Cross-hatched areas represent primary airflow. Total box airflow is higher during cooling than healing. Fan operates only at zone temperatures below the adjustable set point.


Constant-air-volume systems

(Optional) reheat Induction

Constant-air-volume systems

FIGURE 11.24л

Schematic diagram of induction VAV box.

Fan — powered series: Fan is always on and space flow is constant. Damper controls supply of primary air. Perimeter zones may need baseboard or fan-coil units.

Fan-powered parallel: Fan injects plenum air to reheat. Supply pressure drives primary flow that is controlled by dumpers. Variable space flow => less fan energy.

Advantages of VAV: O

Disadvantages and problems of VAV for off design (low flow rate): O

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning

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